1. Field of the Invention
The present invention relates to a sheet feeder, and more particularly, to a sheet feeder of the type which utilizes, as a transporting force (a feeding force), travelling vibration waves, and which is employed in various types of machines provided with a sheet feeding mechanism, such as computers, copying machines, facsimiles, word processors, or typewriters.
2. Related Background Art
Various sheet feeding devices of the type which utilize, as a feeding force, travelling vibration waves formed in an elastic member have been proposed. For example, Japanese Patent Laid-Open No. 59-177243 discloses a sheet feeding device which feeds sheets utilizing travelling waves formed in elastic members for holding the sheet.
FIG. 5 illustrates the principle of the sheet feeding operation made by the aforementioned sheet feeding device.
In FIG. 5, a sheet 12 is held under adequate pressure between two elastic members 10 and 11. In the elastic members 10 and 11, travelling flexural vibrations (progressive waves or vibration waves) are formed such that these two progressive waves are spatially out of phase by 180 degrees. Consequently, the flexural vibrations formed in the individual elastic members 10 and 11 propagate with convex portions thereof opposing the sheet 12. At this time, a surface particle 10a of the convex portion on the surface of the elastic member 10 or 11 generally moves in an elliptical form. For example, when the progressive wave propagates in the elastic member 10 in the direction indicated by an arrow 15 to the right, as viewed in FIG. 5, the mass particle 10a moves clockwise in an elliptical form. Therefore, the direction of movement of the surface particle 10a of the convex portion is reversed to propagation of the vibrations in either of the elastic members 10 and 11. This reversed motion acts as the conveying force of the sheet 12.
On the concave portion, the sheet conveying force is generated in the same direction as the direction of propagation of the vibrations. However, since the pressure in the concave portion is smaller than that generated in the convex portion, friction between the sheet 12 and the elastic members 10 and 11 is small, and the sheet conveying force is thus small. As a result, the sheet conveying forces act as a whole in the reverse direction to the direction of propagation of the flexural vibrations.
The sheet feeding device, shown in FIG. 5, which utilizes a vibration wave has an advantage over the sheet feeding device of the type in which a driving force from, for example, a motor is transmitted to a paper feeding roller through a transmission mechanism, such as a gear train, in that transmission loss of the gear train or the like is eliminated. This allows for a highly accurate sheet feeding.
However, the sheet feeding device shown in FIG. 5 suffers from a disadvantage in that a sheet feeding accuracy may deteriorate due to changes in the environment, such as temperature or humidity, or by non-uniform thickness of the sheets. For example, when sheets are fed through a predetermined distance and then feeding is stopped for printing, deterioration in the sheet feeding accuracy may lead to non-uniform printing.
Regarding a high sheet feeding accuracy of the sheet feeding device, it is also essential that the sheet be precisely stopped at a predetermined position.
In the sheet feeding device shown in FIG. 5, a sheet transporting force is eliminated by suspending generation of the travelling wave formed in the elastic members 10 and 11. Consequently, feeding of the sheet is suspended due to friction between the elastic members 10 and 11 and the sheet.
However, when the sheet feeding speed is large, the sheet is stopped beyond the predetermined position due to the inertia force of the sheet.